The quantitative analysis of SPECT data requires an
accurate determination of the collimator point spread function
(PSF). The aim of this work is to characterize the PSFs of fan
beam and parallel collimators by using Monte Carlo
simulation. Given a particular collimator configuration, a
detailed hexagonal hole array is generated and information
describing its geometry is stored in a look-up table. When a
photon crosses the collimator front plane, a forty-hole array is
placed around its impact position using this table. Each photon
is then tracked up to the detector surface by using the Monte
Carlo code PENELOPE and its associated geometry handling
routines. Particle counters are defined that score the
probability of impact on the detector as a function of the final
photon position. Four sets of counters are employed so as to
differentiate contributions to the geometric, septal penetration,
coherent (Rayleigh) and incoherent (Compton) scatter
components. Furthermore, sensitivity quantification and pulseheight
energy spectra are calculated for different source
locations. Monte Carlo results have been compared with
sensitivity values obtained experimentally and good agreement
was found. Our results show that for 99”Tc imaging, the
geometric component represents about 95% of the fan beam
PSF, whereas the incoherent scattering component is
negligible.